The present invention relates to a semiconductor memory device, more particularly, to a semiconductor memory device wherein memory cells are formed of transistors and so-called groove-type capacitors.
In general, in a dynamic-type semiconductor memory device, each memory cell is comprised by a single MOS transistor and a single capacitor. Such a memory cell enables improved integration of the circuit due to the low number of elements used and, therefore, increased storage density. However, the capacitance of each memory cell preferably should be as large as possible since it is necessary to hold stored data stably for as long a period as possible, and large cell capacitance is also required for increasing the immunity against soft error caused by the radiation of alpha particles. Therefore, to further increase the degree of integration of a semiconductor memory device using such memory cells and, therefore, the storage density, it is necessary to use capacitors offering large capacities with small occupied areas as the capacitors of the memory cells.
In a conventional dynamic-type semiconductor memory device, each memory cell is formed of a transfer gate MOS transistor and a capacitor. The capacitor is a so-called groove-type capacitor and is comprised by providing a V-shaped or U-shaped groove on a semiconductor substrate, attaching a thin insulation film on the inside surface of the groove, and forming a polycrystalline silicon layer on the insulation film. The capacitor for storing data includes an inversion layer which is formed along the groove and which is electrically coupled to the source of the transfer gate MOS transistor and the polycrystalline silicon layer, so that a large capacitance is realized in a small cell area.
However, in the above-mentioned conventional memory device, when the distance between the capacitor grooves of adjoining memory cells is too small, the depletion layer formed under the grooves overflows the entire region between the grooves. Therefore, in this case, the punchthrough phenomenon easily occurs in this region and cells become electrically connected, thereby causing the stored information to be destroyed. Thus, in the conventional memory device, it is impossible to reduce the distance between the grooves of the adjacent memory cells and thus to increase the degree of integration of the device.